Boiler heating zone not working properly

I have an 18 month old PurePro Trio boiler with the Trio Energy Manager Plus. Forced hot water heat. Priority is OFF.

zone 1: hot water tank
zone 2: 1st floor heat
zone 3: 2nd floor heat

Zone 1 works fine.
Zone 2 works fine.

Zone 3 has trouble. When it calls for heat, the zone pumps water, but the boiler will never kick in as it cools down. If zone 3 is the only one calling for heat, it will pump and pump until the boiler gets cold. As soon as another zone calls for heat, the boiler will then fire up just about immediately.

thaks everyone

Thanks for the pointers... I'm going to check everything and make sure this zone is connected just like the others... if so, then I will try to move things to zone 4 instead... perhaps something is wrong with 3.

Water heat is already its own zone. I don't see a need to turn on priority - usually we never get low on hot water or anything, and the boiler is plenty large enough for this house... Any reason I SHOULD turn it on?